Question

One mole of soda ash dissolves in water with evolution of 25 kJ of heat. Dissolution of 1 mol of the hydrate crystals in water resulted in the absorption of 67 kJ of heat. The heat released due to hydration of soda ash is

• A. - 92 kJ
• B. + 42 kJ
• C. - 42 kJ
• D. -102 kJ

Answer 1

The correct option is A - 92 kJ

Given- ${△}_{soln}=-25kJ$, $△H_l=67kJ$
From Hess's law
${△}_{soln}=△H_l+△H_{hydration}$
$-25=+67+△H_{hydration}$
$\Rightarrow △H_{hydration}=-92kJ$